Code Quality

This skill provides code quality principles and best practices for reviewing and improving implementations.

SOLID Principles

Single Responsibility Principle (SRP)

A class/function should have one reason to change.

Bad:

class UserService {
  createUser(data) { /* creates user */ }
  sendEmail(user) { /* sends email */ }
  generateReport(users) { /* generates report */ }
}

Good:

class UserService {
  createUser(data) { /* creates user */ }
}
class EmailService {
  sendEmail(user) { /* sends email */ }
}
class ReportService {
  generateReport(users) { /* generates report */ }
}

Open/Closed Principle (OCP)

Open for extension, closed for modification.

Bad: Adding new payment types requires modifying existing code

function processPayment(type, amount) {
  if (type === 'credit') { /* ... */ }
  else if (type === 'debit') { /* ... */ }
  // Must modify to add new types
}

Good: New payment types can be added without modification

interface PaymentProcessor {
  process(amount: number): Promise<void>;
}
class CreditProcessor implements PaymentProcessor { /* ... */ }
class DebitProcessor implements PaymentProcessor { /* ... */ }

Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types.

Bad:

class Bird {
  fly() { /* ... */ }
}
class Penguin extends Bird {
  fly() { throw new Error("Can't fly!"); } // Violates LSP
}

Good:

class Bird { /* ... */ }
class FlyingBird extends Bird {
  fly() { /* ... */ }
}
class Penguin extends Bird { /* no fly method */ }

Interface Segregation Principle (ISP)

Clients shouldn't depend on interfaces they don't use.

Bad:

interface Worker {
  work(): void;
  eat(): void;
  sleep(): void;
}

Good:

interface Workable { work(): void; }
interface Eatable { eat(): void; }
interface Sleepable { sleep(): void; }

Dependency Inversion Principle (DIP)

Depend on abstractions, not concretions.

Bad:

class UserService {
  private db = new PostgresDatabase();
}

Good:

class UserService {
  constructor(private db: Database) {}
}

DRY, KISS, YAGNI

DRY (Don't Repeat Yourself)

Every piece of knowledge should have a single representation.

Apply when:

• Same logic appears 3+ times
• Changes to one place require changes to others
• Bug fixes need to be applied in multiple places

Don't over-apply:

• Two similar things might diverge later
• Premature abstraction can be worse than duplication

KISS (Keep It Simple, Stupid)

Prefer simple solutions over clever ones.

Simple code:

• Easy to read and understand
• Easy to debug
• Easy to modify
• Has fewer bugs

YAGNI (You Aren't Gonna Need It)

Don't add functionality until it's needed.

Avoid:

• Building for hypothetical requirements
• Adding "just in case" features
• Over-engineering for scale you don't have

Clean Code Principles

Meaningful Names

// Bad
const d = new Date();
const arr = users.filter(u => u.a > 18);

// Good
const currentDate = new Date();
const adultUsers = users.filter(user => user.age > 18);

Small Functions

• Do one thing well
• Few parameters (ideally 0-3)
• Single level of abstraction

Avoid Side Effects

// Bad: side effect hidden in getter
getUser() {
  this.lastAccess = Date.now(); // Side effect!
  return this.user;
}

// Good: explicit about what it does
getUser() {
  return this.user;
}
recordAccess() {
  this.lastAccess = Date.now();
}

Error Handling

// Be specific about errors
class ValidationError extends Error {}
class NotFoundError extends Error {}
class AuthorizationError extends Error {}

// Handle at appropriate level
try {
  await processOrder(order);
} catch (error) {
  if (error instanceof ValidationError) {
    return res.status(400).json({ error: error.message });
  }
  if (error instanceof NotFoundError) {
    return res.status(404).json({ error: error.message });
  }
  throw error; // Re-throw unexpected errors
}

Testing Strategies

Test Pyramid

      /\
     /  \      E2E Tests (few)
    /────\
   /      \    Integration Tests (some)
  /────────\
 /          \  Unit Tests (many)
/────────────\

Unit Testing

• Test individual functions/classes
• Mock dependencies
• Fast and isolated

describe('calculateTotal', () => {
  it('sums item prices', () => {
    const items = [{ price: 10 }, { price: 20 }];
    expect(calculateTotal(items)).toBe(30);
  });

  it('applies discount', () => {
    const items = [{ price: 100 }];
    expect(calculateTotal(items, { discount: 0.1 })).toBe(90);
  });

  it('handles empty cart', () => {
    expect(calculateTotal([])).toBe(0);
  });
});

Integration Testing

• Test component interactions
• Use real dependencies (or test doubles)
• Database, API integration

Test Behavior, Not Implementation

// Bad: tests implementation details
it('calls _internalMethod', () => {
  const spy = jest.spyOn(service, '_internalMethod');
  service.doThing();
  expect(spy).toHaveBeenCalled();
});

// Good: tests behavior
it('sends welcome email on registration', async () => {
  await service.registerUser({ email: 'test@test.com' });
  expect(emailSent).toContainEqual({
    to: 'test@test.com',
    template: 'welcome'
  });
});

Edge Cases to Test

• Empty inputs
• Null/undefined
• Boundary values
• Error conditions
• Concurrent operations
• Large inputs

Code Review Checklist

Correctness

• Does the code do what it's supposed to?
• Are edge cases handled?
• Are error conditions handled?

Security

• Is input validated?
• Are outputs escaped?
• Are secrets protected?

Performance

• Are there N+1 queries?
• Are there unnecessary loops?
• Is caching used appropriately?

Maintainability

• Is the code readable?
• Are names meaningful?
• Is complexity reasonable?
• Is there appropriate documentation?

Testing

• Are there tests?
• Do tests cover edge cases?
• Are tests maintainable?

Common Code Smells

Refactoring Techniques

Extract Function

When a code block can be grouped and named

Inline Function

When the function body is as clear as the name

Extract Variable

When an expression is complex

Rename Variable/Function

When the name doesn't communicate intent

Replace Conditional with Polymorphism

When you have repeated switch/if statements on type

Introduce Parameter Object

When several parameters travel together